The present invention relates to a spectrum analyzer provided with a function whereby the degree of attenuation provided by the input attenuator of the spectrum analyzer can be automatically set to an optimum value with regard to the prevention of generation of spurious signal components within the spectrum analyzer due to distortion, i.e. primarily due to harmonic distortion resulting from nonlinearity of the mixer frequency conversion characteristic when a large-amplitude signal is applied.
The mixer of the frequency converter circuit in a spectrum analyzer, which converts a range of frequency components of an input signal to an intermediate frequency in response to a sweep frequency signal applied from a local oscillator, does not have an ideally linear frequency conversion characteristic. This fact will not normally present a problem when relatively large-amplitude components of the frequency spectrum of an input signal under measurement are being examined on the spectrum analyzer display screen. However when examining low-level components of the spectrum, i.e. low-amplitude peaks on the displayed spectrum, difficulties can arise. That is, if the spectrum contains one or more high-level frequency components, then these can produce harmonic distortion or other types of distortion within the spectrum analyzer circuits, and particularly within the mixer. Such distortion results in the appearance of spurious signal components on the display, which have a similar form to actual input signal frequency component peaks, and may be of comparable or larger amplitude on the display. In the prior art, it has been necessary for the operator of the spectrum analyzer to eliminate such spurious components by manually reducing the amplitude of the input signal applied to the mixer, i.e. by operating a knob controlling an input attenuator disposed between the input signal source and the mixer input. However it is necessary for the operator to be sufficiently knowledgable and experienced to be capable of distinguishing between actual input signal components and spurious components on the display, and this requires many years to acquire.
In order to overcome this problem, it has been proposed to provide a power-level sensor in the input circuit of an spectrum analyzer, and to control the setting of the input attenuator automatically in accordance with the output obtained from this sensor. However, although this method is satisfactory for measuring signals of relatively low frequency, it cannot be employed for measuring high-frequency signals, e.g. of the order of 2 GHz, due to the difficulty of implementing a suitable input circuit and power sensor.
There is therefore a requirement for a spectrum analyzer provided with a function whereby the degree of attenuation provided by an input attenuator can be automatically set to an optimum value, so that the generation of spurious signal components within the spectrum analyzer due to large-amplitude frequency components in the input signal will be prevented, when examining low-amplitude frequency components of the input signal, with no manual adjustment or judgement of the displayed spectrum being required of the operator.
With a prior art spectrum analyzer, a switch is provided whose setting determines the amplitude of an input signal frequency component represented by a specific scale graduation of the display screen. That is to say, if this switch is set such a particular reference graduation, (e.g. the highest scale graduation of the "amplitude" scale of the spectrum analyzer display device) represents +25 dBm, then an input signal frequency component peak of +25 dBm will be displayed on the screen to extend upwards as far as that highest graduation. Such a signal level will be referred to in the following as the "reference level". It will be apparent that the reference level is determined by the overall circuit gain, from the point at which an input signal is applied to the spectrum analyzer to the point at which the intermediate frequency signal is detected, to derive an analog signal representing a frequency spectrum. Thus, the reference level can be altered by varying the degree of attenuation provided by an attenuator disposed between the input signal source and the mixer stage of the spectrum analyzer, or by varying the intermediate frequency amplifier circuit gain. However, in general, the reference level is varied by switching the input attenuator, with the resultant change in reference level being indicated either by the change in switch knob position or by an indication provided on the display screen or elsewhere on the spectrum analyzer. With a prior art spectrum analyzer, therefore, in order to examine a low-level frequency component of an input signal spectrum, the operator will lower the reference level, i.e. will operate this switch such as to reduce the degree of attenuation provided by the input attenuator. However as described above, this will result in spurious signal components being generated, if large-amplitude frequency components are also present in the input signal.